The modification of drugs with poly(ethylene glycol) is a well-established process that improves their pharmacological and biological properties.
Protein and peptide drugs often have a short circulatory half-life in vivo, can have high immunogenicity, can undergo proteolytic degradation, and can have low solubility. Also, prolonged maintenance of therapeutically active drugs in the circulation is a desirable feature of obvious clinical importance.
An attractive strategy for improving the clinical properties of protein or peptide drugs is a modification of the drugs with hydrophilic polymers e.g., polyalkylene oxides (Roberts et al., Adv. Drug Rev. 54, 459-476 (2002)) or polysaccharides, like polysialic acid (Fernandes et al., Biochim. Biophys. Acta, 1341, 26-34 (1997)), dextrans, or hydroxyalkyl starch. Although the modification of protein and peptide drugs with poly(ethylene glycol) (PEG) improves the stability and solubility of the protein or peptide, it often leads to reduced activity. However, subsequent release of PEG moieties from a PEGylated protein or peptide in vivo restores the activity of the protein or peptide. Thus, derivatization of proteins and peptides with releasable PEGs may convert a protein to a controlled-release prodrug with an enhanced circulatory lifetime. These improved biological properties have been shown, for example, in the case of interferon α-2 (Peleg-Shulman et al., J Med Chem. 47, 4897-4904 (2004)), exendin-4 (Tsubery et al., J Biol Chem. 37, 38118-38124 (2004)) and interferon-β-1b (Zhao et al., Bio-conjugate Chem. 17, 341-351 (2006)).
Drug molecules different from proteins and peptides also benefit from PEGylation. PEGylation of drug molecules increases the apparent size of the molecule, thus reducing renal filtration and altering biodistribution. In addition, the PEGylation of hydrophobic ligands increases their solubility in vivo. Finally, derivatization of drug molecules different from proteins and peptides with releasable PEGs provides a method of converting the drug into a controlled-release prodrug.
Several releasable linkers comprising PEG moieties have been suggested. U.S. Pat. No. 6,515,100 describes PEG and related polymer derivatives having weak, hydrolytically unstable linkages to proteins or peptides. However, hydrolysis of the unstable linkage to release PEG from the protein or peptide fails to provide the protein or peptide in its native form. Instead, the protein or peptide comprises an additional short molecular fragment, or tag.
U.S. Pat. No. 7,205,380 describes PEG derivatives, having sterically hindered linkages, that couple with alcohol or thiol groups of proteins or peptides to result in ester or thioester bonds with decreased hydrolytic reactivity. This decreased hydrolytic activity results from the conjugation of alkyl or aryl groups to the carbon adjacent to the carbonyl carbon of the ester or thioester linkage. The patent also discloses that hydrolytic delivery of drugs from PEG esters can be controlled by controlling the number of linking methylene groups in a spacer between the terminal PEG oxygen and the carbonyl group of the attached carboxylic acid or carboxylic acid derivative. The PEG linker of the patent is not optimal with amino-containing proteins because the resulting amide would be more stable to hydrolysis and less able to release PEG from the protein.
U.S. Pat. Nos. 6,413,507 and 6,899,867 describe hydrolytically degradable carbamate derivatives of poly(ethylene glycol). The PEG moiety is conjugated to the protein through a nonhydrolyzable linker attached to an aryl group, which is linked to the protein through a carbamate.
International Publication Nos. WO 04/089280 and WO 06/138572 describe hydrolyzable fluorene-based PEG constructs.
Greenwald et al., (J Med Chem. 42, 3657-3667 (1999)) outlines a general methodology for synthesizing PEG prodrugs of amino-containing compounds. PEG conjugates are described, which follow a double prodrug strategy that relies, first, on enzymatic separation of PEG, followed by a rapid 1,4- or 1,6-benzyl elimination reaction, releasing the conjugated amino-containing drug bound in the form of a carbamate.